ACARA v9 CONTENT DESCRIPTION “classify matter as elements, compounds or mixtures and compare different representations of these, including 2-dimensional and 3-dimensional models, symbols for elements and formulas for molecules and compounds”
Builds on the particle model from Year 7, where all matter was pictured as tiny moving particles. Here we ask a new question about those particles: what kind of atoms are they, and are they bonded together or just mixed? The answer sorts every material into elements, compounds and mixtures.
Three ways matter is put together
All matter can be classified into three groups. An element is made of just one kind of atom. A compound is two or more different elements chemically bonded in a fixed ratio, with properties unlike the elements it formed from. A mixture is two or more substances that are together but not bonded, so they can be separated again by a physical method.
Element, compound or mixture?
Switch the sample and look at the particles. What kind of atoms are there, and are they bonded or free?
An element is made of only one kind of atom. Here every circle is the same colour, and the atoms are not joined to anything different. Iron, oxygen and carbon are elements.
A molecule is not the same as a compound
A molecule is any group of atoms joined together. If those atoms are all the same element, such as the two oxygen atoms in O2, the molecule is still an element. Only when different elements are bonded, as in water, do you have a compound. Comparing O2 with H2O at the particle level makes the difference clear.
Element molecule vs compound molecule
Both are molecules, but only one is a compound. Build each and compare the atoms inside.
Oxygen gas is a molecule of two oxygen atoms joined together. Because both atoms are the same element, O2 is still an element, not a compound. A molecule is not the same as a compound.
Mixtures separate, compounds do not
The simplest test of the difference between a mixture and a compound is whether a physical step can take it apart. In a mixture the substances keep their own particles, so filtering, evaporating or a magnet can sort them out. In a compound the atoms are chemically bonded, so only a chemical reaction can split it.
Can you separate it?
A mixture can be split apart by a physical method. A compound cannot. Choose a sample, then try to separate it.
The two substances are mixed but not bonded. Because each keeps its own particles, a physical method can pull them apart again. Try separating them.
Many ways to picture one substance
Scientists show the same substance in several ways, and each carries different information. A 2D structural diagram shows which atoms are bonded. A 3D model shows the real shape and sizes. Element symbols name the atoms present. A chemical formula packs it all into a few characters. Being able to move between these representations is a core chemistry skill.
One substance, many representations
These are four ways to show the same substance, water. Switch between them and notice that the substance never changes.
A 2D structural diagram flattens the molecule onto the page. The lines stand for the chemical bonds, so you can see that each hydrogen is joined to the central oxygen.
Reading a chemical formula
A chemical formula tells you exactly which atoms make up a molecule and how many of each. Each capital letter is an element symbol, and the small number after a symbol, the subscript, counts how many of that atom there are. When no number is written, there is one. So H2O means two hydrogen atoms and one oxygen atom joined into a single molecule.
Read a formula
A formula is a recipe: each symbol is an element, each subscript is how many of that atom. Pick one and count.
Read the subscript after each symbol to find how many of that atom there are, and read 1 when no number is written. H2O is water: it has 2 H and 1 O, so 3 atoms make up one molecule.
Why this matters
Sorting matter into elements, compounds and mixtures, and reading the symbols and formulas that describe them, is the language of chemistry. Every chemical reaction, every material and every later topic on bonding and reactions depends on being able to classify a substance and read what it is made of.
Quick self-check
1. A substance is made of only one kind of atom. It is a...
2. Which best describes a compound?
3. Salt dissolved in water can be separated by evaporating the water. This tells you salty water is a...
4. A 2D diagram, a 3D ball model and the formula H2O all show...
5. How many atoms in total are in one molecule of CO2?